Glass having refractive index distribution

ABSTRACT

A process for producing a glass having a rod refractive index distribution, which includes pressing a glass at a temperature below the glass transition temperature to form a glass having a density increased towards the surface layer thereof, or alternatively includes heating a glass at a temperature around the transition temperature at the lowest to prepare glass having a uniformly enhanced density and heating the treated glass under a pressure lower than the applied pressure at a temperature below the glass transition temperature of the glass to prepare a rod glass having a density increased towards the central portion.

This is a division of application Ser. No. 08/013,554, filed Dec. 16,1992, now U.S. Pat. No. 5,261,938, which is a continuation ofapplication Ser. No. 07/773,819, filed on Oct. 9, 1991 and nowabandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a process for producing a rod glasshaving an arbitrary refractive index distribution.

For example, in the conventional multimode optical fiber, the presenceof a discontinuous difference in the refractive index between the coreand the clad has frequently caused a mode transformation and a largeloss in the bending of the optical fiber.

This problem has been solved through the use of an optical fiber whereinthe refractive index of the core distributes in a parabolic manner.

Examples of the method of distributing the refractive index in aparabolic manner known in the art include a method wherein the core islaminated by the CVD process while changing the components of thereaction gas to distribute the refractive index, a method wherein aglass rod doped with an ion such as Tl⁺ having a large electronpolarizability is immersed in a melt of KNO₃ or the like to cause ionexchange between Tl⁺ and K⁺, thereby distributing the concentration ofthe dopant ions in a parabolic manner, and a method wherein the ionconcentration is distributed by means of an electric field.

In the CVD process, however, the distribution is provided by controllingthe flow rate of the reaction gas, which requires a very high level oftechnique.

In the method wherein use is made of the ion exchange, it is necessaryto use a combination of ions having different diffusion rates, whichrequires much time and, at the same time, makes it difficult to controlthe distribution. Further, in the method of distributing the ions bymeans of an electric field through the change in the composition of theglass, it is difficult to provide a gradient electric field, so that theprovision of a refractive index distribution as desired becomes verydifficult.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a process which enablesa rod glass having an arbitrary refractive index distribution to beeasily produced.

The above-described object of the present invention can be attained by arod process for producing a glass having a refractive index distributionwhich comprises pressing a glass at a temperature below the glasstransition temperature to form a rod glass having a density increasedtowards the surface layer thereof.

Further, the above-described object can be attained also by pressing aglass at a temperature around the glass transition temperature at thelowest to produce a treated glass having a uniformly enhanced densityand heating the treated glass at a temperature below the glasstransition temperature of said glass under a pressure lower than thatused for the pressing to lower the density of the peripheral portion,thereby producing a rod glass having a density increased towards thecentral portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a refractive index distribution of a quartzglass prepared in the first process of the present invention; and

FIG. 2 is a graph showing a refractive index distribution of a quartzglass prepared in the second process of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process of the present invention is classified into two typesdepending upon the temperature used in the pressing of the glass.

Specifically, in the first process, the glass is pressed at atemperature below the glass transition temperature (hereinafterabbreviated to "Tg").

In the second process, the glass is pressed at a temperature around theTg or above the Tg, that is, at a temperature around the Tg at thelowest. High pressure apparatuses, such as anvil type and belt typeapparatuses, and hot isostatic pressing (HIP) apparatuses may be used asthe pressing apparatus. The glass to be pressed may be any type as faras the composition is homogeneous. However, glasses less liable tocrystallization, such as quartz glass (Tg: about 1100° C.) and BK7 (Tg:about 560° C.), are preferred.

There is no particular limitation on the form of the glass to bepressed. However, the glass should be uniformly pressed from theperiphery. The pressure to be applied varies depending upon the highpressure apparatus. Specifically, in the anvil type and belt type highpressure apparatuses, the pressure to be applied is 50 GPa at thehighest, while in the HIP apparatus, the pressure to be applied is about1 GPa at the highest. The apparatus is selected according to the maximumrefractive index value in the necessary refractive index distribution.

In order to prepare a treated glass less susceptible to change with timeand having a good transparency, it is desired that the Tg and thetreating temperature (Tp) having the following relationship:

0.7≦Tp/Tg≦1.0 (unit of temperature: K) When Tp/Tg is less than 0.7, thetemperature range wherein the glass can be stably used is narrow. On theother hand, when the value is larger than 1.0, the precipitation of acrystal occurs during the pressing treatment depending upon the type ofthe glass, which causes the transparency to be lowered.

In the pressing and heating in the first process of the presentinvention, the glass may be first pressed by means of a pressureapparatus and then heated for a short period of time at a temperaturebelow the Tg. Alternatively, the glass may be first heated at atemperature below the Tg and then pressed.

Since the viscosity of the glass is high in a region of temperaturesbelow the glass transition temperature, the pressure applied to theglass to be pressed is not uniform and decreases from the surfaceportion towards the center portion.

However, the glass has a certain degree of viscosity even at atemperature below the Tg, so that the difference in the pressure betweenthe surface portion and the central portion in the glass to be pressedbecomes small with time. For example, in a quartz glass having a size of1 cm³, the pressure becomes substantially uniform under conditions of atemperature of 0.9 in terms of Tp/Tg and a holding time of about 5 minalthough it depends upon the type, size, Tp/Tg and intended gradientrefractive index. Therefore, before the difference in the pressurebetween the surface portion and the central portion becomes zero, thatis, after the pressing for 5 min or less in the above-described quartzglass, the pressed glass is cooled to room temperature to prepare aglass having a density increased towards the surface portion, that is, aglass having a refractive index increased towards the surface portion.

When the treating .pressure is varied within a given treating time, thegradient refractive index of the resultant glass increases withincreasing the pressure and decreases with increasing the treating time.

According to the above-described second process of the presentinvention, the nonuniformity of the pressure applied to the glass isalleviated by heating the starting glass having a homogeneouscomposition pressed by means of a high pressure apparatus for asufficient period of time around the Tg or a predetermined range oftemperatures above the Tg, or conducting the pressing treatment for asufficient period of time while heating the glass at a temperaturearound the Tg. This provides a treated glass having a uniformly enhanceddensity free from the difference in the density between the surfacelayer portion and the central portion, that is, having a refractiveindex higher than that before the treatment. Since the variation in thetreating pressure brings about a variation in the refractive index andthe refractive index of the resultant glass increases with increasingthe pressure, it is possible to prepare a uniformly treated glass havingan arbitrary refractive index higher than that before the treatment.

In the second process of the present invention, the heat treatment isconducted under a pressure lower than the treating pressure used for thetreated glass at a temperature below the Tg. This lowers the density ofthe treated glass from the surface portion towards the central portion.In this case, the lowering rate of the density decreases withapproaching the central portion, and the central portion remains at ahigh density. Accordingly, when the cooling is conducted in this state,there is obtained a glass having such a refractive index distributionthat the density increases towards the central portion.

In all the glasses prepared by the above-described first process andsecond process, the composition is homogeneous and there occurs nodifference in the transmittance property although the refractive indexdistribution occurs due to the pressure. Specifically, the glass of thepresent invention is characterized by a uniform transmittance as opposedto the conventional distribution glass wherein the transmittance variesfrom site to site.

The present invention will now be described in more detail by referringto the following Examples.

EXAMPLE 1

A quartz glass having a size of 4 cm square was subjected to a hotisostatic pressing (HIP) treatment under a pressure of 150 MPa at atemperature of 1000° C. for 30 min. The resultant block of quartz glasswas cut to measure the refractive index distribution. The results aregiven in FIG. 1 in terms of the relationship between the refractiveindex, n, and the distance, L, from the center of the block. Therefractive index decreased from the surface portion (1.4632) towards theinternal portion (1.4626), and the change in the refractive index was0.04%.

EXAMPLE 2

A quartz glass having a size of 4 cm square was subjected to a hotisostatic pressing (HIP) treatment under a pressure of 150 MPa at atemperature of 1200° C. for 2 hr to prepare a homogeneous treated glass.Thereafter, the glass was maintained in the air at 900° C. for 30 min.As with FIG. 1, the refractive index distribution of the resultantquartz glass is shown in FIG. 2. The refractive index increased from thesurface portion (1.4613) towards the internal portion (1.4634), and thechange in the refractive index was 0.14%.

As described above, according to the present invention, a glass having arefractive index distribution can be prepared in a short time.

Specifically, (1) a treated glass having a refractive index increasedtowards the surface portion can be prepared by pressing a glass having ahomogeneous composition by means of a high pressure apparatus whileheating the glass at a temperature below the glass transitiontemperature. For example, a rod lens having the same function as that ofa concave lens is obtained from a cylindrical glass.

(2) When the glass is first pressed around the Tg or at a temperatureabove the Tg until the density becomes uniform and then cooled, it ispossible to prepare a homogeneous treated glass having an arbitraryrefractive index higher than that before the treatment through thevariation in the treating conditions.

(3) When the treated glass prepared in the above item (2) isheat-treated again under a pressure lower than that used for theproduction of the above-described treated glass at a temperature belowthe Tg, there occurs a thermal relaxation of the surface, so that aglass having such a refractive index distribution that the refractiveindex at the central portion is high while the refractive index at thesurface portion is low. For example, a rod lens having the same functionas that of a convex lens is prepared from a cylindrical glass.

Therefore, in any case, it is possible to prepare a glass having anarbitrary refractive index distribution. In particular, it is possibleto eliminate the difference in the transmittance between sites whilesubstantially maintaining the optical characteristics which has beenunattainable by the conventional CVD process or ion exchange process.

What is claimed is:
 1. A rod glass article having a central portion andan outer portion, wherein said central portion has a higher refractiveindex than said outer portion, the composition of said rod glass ishomogeneous, and the increase in said refractive index is uniform.
 2. Arod glass article having a refractive index distribution and producedaccording to a method comprising pressing a starting glass having auniform composition at its glass transition temperature or above it toproduce a treated glass having a uniformly higher refractive index thanthe starting glass and heating the treated glass under a pressure lowerthan that as applied above at a temperature below the glass transitiontemperature of said glass to produce a rod glass article having arefractive index increased towards a central portion of said rod glassarticle, wherein the glass transition temperature (Tg) and thetemperature during pressing (Tp) has the following relationship:1.0≦Tp/Tg≦1.3 where Tp and Tg are each expressed in Kelvin, therebyproducing a rod glass article having a uniform composition and a uniformrefractive index distribution.